In the main yes. Most dimmers use SCR or FET's to clip the wave form there are leading and lagging but if you were to use fully half wave rectified the power would be half but the light would be a lot less than half most the power being heat rather than light.
I noted in Turkey where they have used discharge lighting well before us because it hotter they split their lights into two groups 2/3 in one group and 1/3 in other group giving off, 1/3, 2/3, and full just using two switches which is a better way to do it.
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